rustial-renderer-bevy 0.0.1

// ---------------------------------------------------------------------------
//! # Terrain mesh sync system
//!
//! Synchronises terrain meshes produced by the engine's
//! [`TerrainManager`](rustial_engine::TerrainManager) to Bevy mesh
//! entities each frame.
//!
//! ## Entity lifecycle
//!
//! Terrain entities are managed with a **diff-based** strategy keyed
//! on [`TileId`]:
//!
//! 1. **Despawn** -- entities whose `TileId` is no longer in the
//!    engine's desired set are scheduled for despawn.
//! 2. **Reposition** -- entities whose `TileId` *is* still desired
//!    have their [`Transform`] updated so that vertices stay
//!    camera-relative as the user pans (avoids f32 jitter).
//! 3. **Spawn** -- tiles present in the engine but not yet in the ECS
//!    get a fresh `Mesh`, `StandardMaterial`, and `TerrainEntity`
//!    marker component.
//!
//! ### Why not rebuild every frame?
//!
//! Terrain meshes can be large (thousands of vertices with skirts).
//! Uploading a new `Mesh` asset each frame would be wasteful.  Instead,
//! vertex positions are stored in **world space** (f64, from the
//! engine's `TerrainMeshData`) and the camera offset is applied via a
//! `Transform` translation.  This means mesh data is uploaded only
//! once per tile; subsequent frames just update a single `Vec3`
//! translation.
//!
//! ## Coordinate pipeline
//!
//! ```text
//! TerrainMeshData.positions (f64, Web Mercator metres)
//!     |
//!     |  subtract camera_origin  (at spawn time, baked into vertex data)
//!     v
//! mesh-local (f32)  -- small values at spawn, centred near camera
//!     |
//!     |  Transform.translation  (updated every frame)
//!     v
//! camera-relative (f32)  -- accounts for camera drift since spawn
//! ```
//!
//! At **spawn** the camera origin at that instant is subtracted from
//! every vertex and stored in the mesh.  The entity remembers this
//! origin in [`TerrainEntity::spawn_origin`].  Each subsequent frame,
//! the `Transform` translation is set to `spawn_origin - current_origin`
//! so that the net effect is always `vertex - current_origin`.
//!
//! ## Scheduling
//!
//! Registered in [`Update`](bevy::prelude::Update) -- after
//! `PreUpdate` has ticked the engine state and synced the camera.
// ---------------------------------------------------------------------------

use crate::components::DeferredAssetDrop;
use crate::components::TerrainEntity;
use crate::plugin::MapStateResource;
use crate::systems::frame_change_detection::{frame_unchanged, FrameChangeDetection};
use crate::tile_fog_material::{TerrainUniforms, TileFogMaterial};
use bevy::asset::RenderAssetUsages;
use bevy::camera::visibility::NoFrustumCulling;
use bevy::mesh::{Indices, PrimitiveTopology};
use bevy::prelude::*;
use bevy::render::render_resource::{Extent3d, TextureDimension, TextureFormat};
use rustial_engine as rustial_math;
use rustial_engine::{materialize_terrain_mesh, CameraProjection, TerrainMeshData, TileId};
use std::collections::{HashMap, HashSet};

const PROJECTION_WEB_MERCATOR: f32 = 0.0;
const PROJECTION_EQUIRECTANGULAR: f32 = 1.0;

// ---------------------------------------------------------------------------
// Placeholder texture
// ---------------------------------------------------------------------------

/// Shared 1x1 placeholder image for terrain materials.
///
/// Terrain entities are spawned before their map-tile textures are
/// available.  Bevy's `AsBindGroup` pipeline may silently skip
/// rendering a material whose `Option<Handle<Image>>` is `None`
/// because it cannot create a valid bind group.  Providing a concrete
/// (tiny) texture as a placeholder guarantees that the material is
/// always renderable, with the shader's `has_texture = 0` flag
/// steering it to the neutral-tint code path.
#[derive(Resource)]
pub struct TerrainPlaceholderTexture(pub Handle<Image>);

/// Shared 1x1 placeholder image for terrain height textures.
#[derive(Resource)]
pub struct TerrainHeightPlaceholderTexture(pub Handle<Image>);

/// Shared 1x1 placeholder image for hillshade overlay materials.
#[derive(Resource)]
pub struct HillshadePlaceholderTexture(pub Handle<Image>);

/// Shared reusable terrain grid meshes keyed by grid resolution.
#[derive(Resource, Default)]
pub struct SharedTerrainGridMeshes {
    handles: HashMap<u16, Handle<Mesh>>,
}

/// Uploaded terrain height textures keyed by tile and generation.
#[derive(Resource, Default)]
pub struct UploadedTerrainHeightTextures {
    handles: HashMap<(TileId, u64), Handle<Image>>,
}

/// System to initialise the placeholder texture at startup.
pub fn init_placeholder_texture(mut commands: Commands, mut images: ResMut<Assets<Image>>) {
    let image = Image::new(
        Extent3d {
            width: 1,
            height: 1,
            depth_or_array_layers: 1,
        },
        TextureDimension::D2,
        vec![255, 255, 255, 255], // 1x1 white pixel
        TextureFormat::Rgba8UnormSrgb,
        RenderAssetUsages::default(),
    );
    let handle = images.add(image);
    commands.insert_resource(TerrainPlaceholderTexture(handle));

    let height = Image::new(
        Extent3d {
            width: 1,
            height: 1,
            depth_or_array_layers: 1,
        },
        TextureDimension::D2,
        0.0f32.to_le_bytes().to_vec(),
        TextureFormat::R32Float,
        RenderAssetUsages::default(),
    );
    let height_handle = images.add(height);
    commands.insert_resource(TerrainHeightPlaceholderTexture(height_handle));

    let hillshade = Image::new(
        Extent3d {
            width: 1,
            height: 1,
            depth_or_array_layers: 1,
        },
        TextureDimension::D2,
        vec![128, 128, 255, 255],
        TextureFormat::Rgba8UnormSrgb,
        RenderAssetUsages::default(),
    );
    let hillshade_handle = images.add(hillshade);
    commands.insert_resource(HillshadePlaceholderTexture(hillshade_handle));
}

// ---------------------------------------------------------------------------
// Helpers
// ---------------------------------------------------------------------------

pub(crate) fn supports_gpu_terrain_path(
    mesh: &TerrainMeshData,
    projection: CameraProjection,
) -> bool {
    mesh.elevation_texture.is_some()
        && matches!(
            projection,
            CameraProjection::WebMercator | CameraProjection::Equirectangular
        )
}

pub(crate) fn terrain_uniforms_for_mesh(
    mesh: &TerrainMeshData,
    camera_origin: glam::DVec3,
    projection: CameraProjection,
) -> TerrainUniforms {
    let nw = rustial_math::tile_to_geo(&mesh.tile);
    let se = rustial_math::tile_xy_to_geo(
        mesh.tile.zoom,
        mesh.tile.x as f64 + 1.0,
        mesh.tile.y as f64 + 1.0,
    );
    let effective_skirt =
        rustial_engine::skirt_height(mesh.tile.zoom, mesh.vertical_exaggeration as f64) as f32;
    let (skirt_base, min_elev, max_elev) = mesh
        .elevation_texture
        .as_ref()
        .map(|elevation| {
            // Clamp skirt base: don't let extreme ocean data in distant
            // tiles produce skirts that plunge kilometers below the surface.
            // The skirt only needs to hide seams, so cap it at a generous
            // depth below the effective skirt height.
            let raw_base = elevation.min_elev * mesh.vertical_exaggeration - effective_skirt;
            let base = raw_base.max(-effective_skirt * 3.0);
            (base, elevation.min_elev, elevation.max_elev)
        })
        .unwrap_or((0.0, 0.0, 0.0));

    let elev_region = mesh
        .elevation_texture
        .as_ref()
        .map(|elevation| {
            if mesh.tile != mesh.elevation_source_tile {
                rustial_engine::elevation_region_in_texture_space(
                    mesh.elevation_region,
                    elevation.width,
                    elevation.height,
                )
            } else {
                mesh.elevation_region
            }
        })
        .unwrap_or(mesh.elevation_region);

    TerrainUniforms {
        geo_bounds: Vec4::new(nw.lat as f32, nw.lon as f32, se.lat as f32, se.lon as f32),
        scene_origin: scene_origin_uniform(camera_origin, projection),
        elev_params: Vec4::new(mesh.vertical_exaggeration, skirt_base, min_elev, max_elev),
        elev_region: Vec4::new(
            elev_region.u_min,
            elev_region.v_min,
            elev_region.u_max,
            elev_region.v_max,
        ),
        options: Vec4::new(1.0, 0.0, 0.0, 0.0),
    }
}

pub(crate) fn scene_origin_uniform(
    camera_origin: glam::DVec3,
    projection: CameraProjection,
) -> Vec4 {
    let projection_kind = match projection {
        CameraProjection::WebMercator => PROJECTION_WEB_MERCATOR,
        CameraProjection::Equirectangular => PROJECTION_EQUIRECTANGULAR,
        _ => PROJECTION_WEB_MERCATOR,
    };
    Vec4::new(
        camera_origin.x as f32,
        camera_origin.y as f32,
        camera_origin.z as f32,
        projection_kind,
    )
}

pub(crate) fn get_or_create_shared_grid_mesh_handle(
    meshes: &mut Assets<Mesh>,
    shared_grids: &mut SharedTerrainGridMeshes,
    resolution: u16,
) -> Handle<Mesh> {
    if let Some(handle) = shared_grids.handles.get(&resolution) {
        return handle.clone();
    }

    let res = (resolution as usize).max(2);
    let (positions, uvs, indices) = build_shared_grid_mesh(res);
    let mut mesh = Mesh::new(PrimitiveTopology::TriangleList, Default::default());
    mesh.insert_attribute(Mesh::ATTRIBUTE_POSITION, positions);
    mesh.insert_attribute(Mesh::ATTRIBUTE_NORMAL, vec![[0.0, 0.0, 1.0]; uvs.len()]);
    mesh.insert_attribute(Mesh::ATTRIBUTE_UV_0, uvs);
    mesh.insert_indices(Indices::U32(indices));

    let handle = meshes.add(mesh);
    shared_grids.handles.insert(resolution, handle.clone());
    handle
}

pub(crate) fn get_or_create_height_texture_handle(
    images: &mut Assets<Image>,
    height_cache: &mut UploadedTerrainHeightTextures,
    mesh: &TerrainMeshData,
    fallback: &Handle<Image>,
) -> Handle<Image> {
    let Some(elevation) = mesh.elevation_texture.as_ref() else {
        return fallback.clone();
    };
    let key = (mesh.tile, mesh.generation);
    if let Some(handle) = height_cache.handles.get(&key) {
        return handle.clone();
    }

    let bytes = bytemuck::cast_slice::<f32, u8>(&elevation.data).to_vec();

    let image = Image::new(
        Extent3d {
            width: elevation.width.max(1),
            height: elevation.height.max(1),
            depth_or_array_layers: 1,
        },
        TextureDimension::D2,
        bytes,
        TextureFormat::R32Float,
        RenderAssetUsages::default(),
    );
    let handle = images.add(image);
    height_cache.handles.insert(key, handle.clone());
    handle
}

fn build_shared_grid_mesh(resolution: usize) -> (Vec<[f32; 3]>, Vec<[f32; 2]>, Vec<u32>) {
    let mut positions = Vec::with_capacity(resolution * resolution);
    let mut uvs = Vec::with_capacity(resolution * resolution);
    let mut indices = Vec::with_capacity((resolution - 1) * (resolution - 1) * 6);

    for row in 0..resolution {
        for col in 0..resolution {
            let u = col as f32 / (resolution - 1) as f32;
            let v = row as f32 / (resolution - 1) as f32;
            positions.push([u, v, 0.0]);
            uvs.push([u, v]);
        }
    }

    for row in 0..(resolution - 1) {
        for col in 0..(resolution - 1) {
            let tl = (row * resolution + col) as u32;
            let tr = tl + 1;
            let bl = ((row + 1) * resolution + col) as u32;
            let br = bl + 1;
            indices.extend_from_slice(&[tl, bl, tr, tr, bl, br]);
        }
    }

    let edges: [Vec<usize>; 4] = [
        (0..resolution).collect(),
        ((resolution - 1) * resolution..resolution * resolution).collect(),
        (0..resolution).map(|r| r * resolution).collect(),
        (0..resolution)
            .map(|r| r * resolution + resolution - 1)
            .collect(),
    ];

    for edge in &edges {
        for i in 0..edge.len() - 1 {
            let a = edge[i] as u32;
            let b = edge[i + 1] as u32;
            let uv_a = uvs[edge[i]];
            let uv_b = uvs[edge[i + 1]];
            let base_a = positions.len() as u32;
            let base_b = base_a + 1;
            positions.push([uv_a[0], uv_a[1], 1.0]);
            positions.push([uv_b[0], uv_b[1], 1.0]);
            uvs.push(uv_a);
            uvs.push(uv_b);
            indices.extend_from_slice(&[a, base_a, b, b, base_a, base_b]);
        }
    }

    (positions, uvs, indices)
}

// ---------------------------------------------------------------------------
// Resource: LastSceneOrigin
// ---------------------------------------------------------------------------

/// Tracks the previous scene origin to avoid unnecessary material updates
/// when the camera hasn't moved.
#[derive(Resource, Default)]
pub struct LastSceneOrigin {
    origin: Option<[i64; 3]>,
}

impl LastSceneOrigin {
    fn quantise(origin: glam::DVec3) -> [i64; 3] {
        [
            (origin.x * 100.0) as i64,
            (origin.y * 100.0) as i64,
            (origin.z * 100.0) as i64,
        ]
    }

    fn changed(&self, origin: glam::DVec3) -> bool {
        let q = Self::quantise(origin);
        self.origin.as_ref() != Some(&q)
    }

    fn update(&mut self, origin: glam::DVec3) {
        self.origin = Some(Self::quantise(origin));
    }
}

// ---------------------------------------------------------------------------
// System
// ---------------------------------------------------------------------------

/// Synchronise engine terrain meshes to Bevy mesh entities.
///
/// See the [module-level documentation](self) for the full entity
/// lifecycle, coordinate pipeline, and repositioning strategy.
#[allow(clippy::too_many_arguments)]
pub fn sync_terrain(
    mut commands: Commands,
    state: Res<MapStateResource>,
    mut existing: Query<(
        Entity,
        &TerrainEntity,
        &mut Transform,
        &Mesh3d,
        &MeshMaterial3d<TileFogMaterial>,
    )>,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<TileFogMaterial>>,
    mut images: ResMut<Assets<Image>>,
    mut deferred: ResMut<DeferredAssetDrop>,
    mut shared_grids: ResMut<SharedTerrainGridMeshes>,
    mut height_cache: ResMut<UploadedTerrainHeightTextures>,
    placeholder: Option<Res<TerrainPlaceholderTexture>>,
    height_placeholder: Option<Res<TerrainHeightPlaceholderTexture>>,
    mut last_origin: ResMut<LastSceneOrigin>,
    detection: Res<FrameChangeDetection>,
) {
    // Whole-frame skip: if nothing in the engine changed, terrain sync
    // has no work to do.
    if frame_unchanged(&detection, &state.0)
        && !(state.0.terrain().enabled()
            && !state.0.terrain_meshes().is_empty()
            && existing.is_empty())
    {
        return;
    }

    let terrain_meshes = state.0.terrain_meshes();
    let camera_origin = state.0.scene_world_origin();
    let projection = state.0.camera().projection();

    let origin_changed = last_origin.changed(camera_origin);

    // -----------------------------------------------------------------
    // Fast path: terrain disabled -> despawn everything.
    // -----------------------------------------------------------------
    if !state.0.terrain().enabled() {
        for (entity, _, _, mesh_handle, mat_handle) in existing.iter() {
            deferred.keep_mesh(mesh_handle.0.clone());
            deferred.keep_material(mat_handle.0.clone());
            commands.entity(entity).despawn();
            log::trace!("terrain_sync: despawned (terrain disabled)");
        }
        last_origin.update(camera_origin);
        return;
    }

    // Terrain is enabled but no meshes are currently available (e.g.
    // transient cache miss while rotating/pitching). Keep existing
    // terrain entities instead of clearing the scene.
    if terrain_meshes.is_empty() {
        if origin_changed {
            for (_, terrain, mut transform, _, material_handle) in existing.iter_mut() {
                if terrain.gpu_displaced {
                    transform.translation = Vec3::ZERO;
                    if let Some(material) = materials.get_mut(&material_handle.0) {
                        material.terrain.scene_origin =
                            scene_origin_uniform(camera_origin, projection);
                    }
                } else {
                    let offset = terrain.spawn_origin - camera_origin;
                    transform.translation =
                        Vec3::new(offset.x as f32, offset.y as f32, offset.z as f32);
                }
            }
            last_origin.update(camera_origin);
        }
        return;
    }

    // -----------------------------------------------------------------
    // Build the desired set of tile IDs from the engine.
    // -----------------------------------------------------------------
    let desired: HashSet<TileId> = terrain_meshes.iter().map(|m| m.tile).collect();
    let engine_generations: HashMap<TileId, u64> = terrain_meshes
        .iter()
        .map(|m| (m.tile, m.generation))
        .collect();
    let engine_gpu_path: HashMap<TileId, bool> = terrain_meshes
        .iter()
        .map(|m| (m.tile, supports_gpu_terrain_path(m, projection)))
        .collect();

    // -----------------------------------------------------------------
    // Phase 1: Despawn entities whose tile is no longer desired OR
    //          whose mesh generation is stale (elevation data arrived
    //          after the entity was spawned with a placeholder mesh).
    // -----------------------------------------------------------------
    for (entity, terrain, _, mesh_handle, mat_handle) in existing.iter() {
        let dominated = !desired.contains(&terrain.tile_id);
        let stale = engine_generations
            .get(&terrain.tile_id)
            .is_some_and(|&generation| generation != terrain.mesh_generation);
        let gpu_path_changed = engine_gpu_path
            .get(&terrain.tile_id)
            .is_some_and(|&gpu| gpu != terrain.gpu_displaced);

        if dominated || stale || terrain.projection != projection || gpu_path_changed {
            deferred.keep_mesh(mesh_handle.0.clone());
            deferred.keep_material(mat_handle.0.clone());
            commands.entity(entity).despawn();
            log::trace!(
                "terrain_sync: despawned tile {:?} (dominated={dominated}, stale={stale}, gpu_path_changed={gpu_path_changed})",
                terrain.tile_id,
            );
        }
    }

    // -----------------------------------------------------------------
    // Phase 2: Reposition surviving entities.
    // -----------------------------------------------------------------
    let mut existing_ids: HashSet<TileId> = HashSet::with_capacity(desired.len());
    for (_, terrain, mut transform, _, material_handle) in existing.iter_mut() {
        if !desired.contains(&terrain.tile_id) {
            // Already scheduled for despawn above -- skip.
            continue;
        }
        // Also skip stale entities scheduled for despawn above.
        if engine_generations
            .get(&terrain.tile_id)
            .is_some_and(|&generation| generation != terrain.mesh_generation)
        {
            continue;
        }
        if terrain.projection != projection {
            continue;
        }
        if engine_gpu_path
            .get(&terrain.tile_id)
            .is_some_and(|&gpu| gpu != terrain.gpu_displaced)
        {
            continue;
        }
        existing_ids.insert(terrain.tile_id);

        if origin_changed {
            if terrain.gpu_displaced {
                transform.translation = Vec3::ZERO;
                if let Some(material) = materials.get_mut(&material_handle.0) {
                    material.terrain.scene_origin = scene_origin_uniform(camera_origin, projection);
                }
            } else {
                let offset = terrain.spawn_origin - camera_origin;
                transform.translation =
                    Vec3::new(offset.x as f32, offset.y as f32, offset.z as f32);
            }
        }
    }

    // -----------------------------------------------------------------
    // Phase 3: Spawn new terrain mesh entities.
    // -----------------------------------------------------------------
    let Some(placeholder) = placeholder else {
        return;
    };
    let Some(height_placeholder) = height_placeholder else {
        return;
    };

    let needed_height_keys: HashSet<(TileId, u64)> = terrain_meshes
        .iter()
        .filter(|mesh| supports_gpu_terrain_path(mesh, projection))
        .map(|mesh| (mesh.tile, mesh.generation))
        .collect();

    for terrain_mesh in terrain_meshes {
        if existing_ids.contains(&terrain_mesh.tile) {
            continue;
        }

        let gpu_displaced = supports_gpu_terrain_path(terrain_mesh, projection);
        let (mesh_handle, material, transform) = if gpu_displaced {
            let mesh_handle = get_or_create_shared_grid_mesh_handle(
                &mut meshes,
                &mut shared_grids,
                terrain_mesh.grid_resolution,
            );
            let height_handle = get_or_create_height_texture_handle(
                &mut images,
                &mut height_cache,
                terrain_mesh,
                &height_placeholder.0,
            );
            let material = TileFogMaterial {
                tile_texture: Some(placeholder.0.clone()),
                flags: Default::default(),
                terrain: terrain_uniforms_for_mesh(terrain_mesh, camera_origin, projection),
                height_texture: height_handle,
                ..TileFogMaterial::default()
            };
            (mesh_handle, material, Transform::IDENTITY)
        } else {
            let materialized = materialize_terrain_mesh(
                terrain_mesh,
                projection,
                rustial_engine::skirt_height(
                    terrain_mesh.tile.zoom,
                    terrain_mesh.vertical_exaggeration as f64,
                ),
            );
            if materialized.positions.is_empty() || materialized.indices.is_empty() {
                log::trace!(
                    "terrain_sync: skipping degenerate mesh for tile {:?}",
                    terrain_mesh.tile,
                );
                continue;
            }

            let positions: Vec<[f32; 3]> = materialized
                .positions
                .iter()
                .map(|p| {
                    [
                        (p[0] - camera_origin.x) as f32,
                        (p[1] - camera_origin.y) as f32,
                        (p[2] - camera_origin.z) as f32,
                    ]
                })
                .collect();

            let mut mesh = Mesh::new(PrimitiveTopology::TriangleList, Default::default());
            mesh.insert_attribute(Mesh::ATTRIBUTE_POSITION, positions);
            mesh.insert_attribute(Mesh::ATTRIBUTE_NORMAL, materialized.normals);
            mesh.insert_attribute(Mesh::ATTRIBUTE_UV_0, materialized.uvs);
            mesh.insert_indices(Indices::U32(materialized.indices));

            let mesh_handle = meshes.add(mesh);
            let material = TileFogMaterial {
                tile_texture: Some(placeholder.0.clone()),
                height_texture: height_placeholder.0.clone(),
                ..TileFogMaterial::default()
            };
            (mesh_handle, material, Transform::IDENTITY)
        };

        let material_handle = materials.add(material);

        commands.spawn((
            Mesh3d(mesh_handle),
            MeshMaterial3d(material_handle),
            transform,
            Visibility::Visible,
            NoFrustumCulling,
            TerrainEntity {
                tile_id: terrain_mesh.tile,
                spawn_origin: camera_origin,
                projection,
                gpu_displaced,
                mesh_generation: terrain_mesh.generation,
                has_exact_texture: false,
            },
        ));

        log::trace!("terrain_sync: spawned tile {:?}", terrain_mesh.tile);
    }

    // Only clean up the height texture cache when it has grown beyond
    // the working set.  During fly-to animations the needed set changes
    // every frame, so pruning every frame is wasteful.  A 2× headroom
    // lets recently-expired entries linger a few frames (cheap in GPU
    // memory: each entry is a single R32Float tile) while avoiding
    // O(cache_size) retain() on every frame.
    let cache_headroom = needed_height_keys.len().max(16) * 2;
    if height_cache.handles.len() > cache_headroom {
        height_cache
            .handles
            .retain(|key, _| needed_height_keys.contains(key));
    }

    last_origin.update(camera_origin);
}